As product designs for minimally invasive surgery become more and more complex, the finishing requirements for stainless tubing are also increasing in complexity.
Many device applications using stainless steel tubing now require improved surface finish on the ID of the tubing.
- This creates smoother inner surfaces that reduce friction for mating parts or parts that must travel in and/or out of the tubing during the surgical procedure.
- There are also an increasing number of fluid dispensing applications where improved surface finish is critical to success. Blood sampling and testing equipment uses very expensive chemical reagents to mix with blood during the testing cycle. This type of carryover reduction can be supported through careful attention to the surface finish in the injection probe and transfer tubing. In these types of applications it is critical to reduce carryover from one sample to another.
- This reduces total cost of the testing (due to a more efficient use of reagents) and improves the accuracy of test results (with more precise amounts of chemicals used in the tests).
The 3 steps for improving surface finish on the ID of tubing are:
1. Define the desired outcome, measurement methods and measurement units. This will assure a clear definition of what improved performance results are needed.
A note on measurement units -> The finish on hypodermic tubing is usually measured in Ra, Rq, or Rms.
Rq = Rms: The Root-Mean-Square
- Rq is the same as RMS is a statistical measurement of the square root of the average of the squares of the measurement. The Rq or Rms value is generally 11% higher than the Ra value for the same surface roughness. Keep in mind that the ~11% difference represents measurements from a normal test patch with a pure sine-wave profile. However, there is actually no mathematical relationship between the two parameters and, depending on the actual manufacturing process and the resulting surface profile, the ratio between Rq and Ra can vary by as much as 200%.
Ra: The arithmetic mean
- The average roughness, Ra, is expressed in units of height. In the Imperial (English) system, Ra typically expressed in “millionths” of an inch. This is also referred to as “microinches” or sometimes just as “micro” (however the latter is just slang). In the metric system, Ra is typically expressed as “millionths of a meter” also called “micrometers” or “microns”.
Cadence can use any of the above units of measurement to generate the desired results.
2. Generate variations of the product with better finish. This can be done via numerous options, such as:
- Micro-Polishing and/or passivation
- Bore enhancement of tubing via use of a “bright draw” process.
- Coatings, such as Silicone, PTFE or PSX Coatings. These may also provide greater lubricity.
- Abrasive Flow Machining or Extrude Hone Process. This process is distinctive in that it effectively provides precision deburring, polishing and edge rounding of internal and external surfaces. Anywhere the unique abrasive laden polymer — or, media — can flow, precision finishing occurs. This process is fairly expensive with the ultimate ID finish improvement a function of the tube ID size and the tube length.
3. Test the variations in the surface finish to correlate actual finish improvement with desired outcome improvement.
Repeat and refine steps #1 through #3 as needed to optimize the final product design!